The present invention relates in general to electronic circuits and, more particularly, to switching power supplies which convert an AC signal to a regulated DC signal.
A DC voltage is typically derived from an AC power source, e.g. by plugging a power supply into a wall socket. The AC voltage available at the wall socket is converted to a DC bulk voltage by a full-wave rectifier diode bridge. The DC bulk voltage is further converted to a regulated DC output voltage by a switching power supply.
The switching power supply uses a transformer, or an inductor depending on the configuration, as an energy transfer element. For example, a flyback-type power supply has a power switching transistor coupled to one side of the primary winding of a transformer. The power transistor turns on and off as determined by a regulator circuit to alternately store energy in the magnetic field of the transformer and transfer the stored energy to the secondary winding. The secondary winding of the transformer develops an output voltage across a shunt capacitor coupled across the secondary winding as a function of the energy transfer. The voltage across the capacitor provides the DC output voltage of the switching power supply.
The DC output voltage increases and decreases with the applied load. An increasing load decreases the DC output voltage and a decreasing load increases the DC output voltage. In most prior art applications, the DC output voltage, or a representation thereof from the secondary side of the transformer, is fed back to the regulator circuit so the switching power supply can compensate for load variation. As the load increases, the DC output voltage decreases causing the regulator to leave the power transistor on for a longer period of time to store more energy in the magnetic field. The additional energy is transferred to the secondary winding during the off time of the power transistor to supply the increased load and re-establish the DC output voltage. As the load decreases, the DC output voltage increases which causes the regulator to leave the power transistor on for a shorter period of time to store less energy in the magnetic field. The reduced energy transfer to the secondary winding during the off time of the power transistor causes the power supply to adjust to the decreased load and reduce the DC output voltage back to its steady-state value.
FIG. 1 illustrates prior art switching power supply 10 that receives an AC line voltage and converts it to a regulated DC operating voltage. Specifically, an AC line voltage is converted to a DC bulk voltage by the full-wave rectifier diode bridge 12. Capacitor 14 filters the DC bulk voltage, and the primary winding of transformer 16 receives the DC bulk voltage. Regulator 18 modulates an inductor current through the primary winding of transformer 16 to control the amount of energy stored in the magnetic field of transformer 16. The energy stored in the magnetic field is transferred to the secondary winding where capacitor 20 is coupled across the secondary winding to develop a DC output voltage (V.sub.OUT). Diode 22 and photo-detection transistor 24 act together to optically couple feedback information from reference and error amplifier 23 to regulator circuit 18 to regulate V.sub.OUT of switching power supply 30.
Most switching power supplys have a feedback mechanism on the secondary side of the transformer. The feedback mechanism provides a feedback signal to a regulator circuit to regulate the output voltage of the switching power supply. The feedback mechanism is typically comprised of optical devices, and a reference and error amplifier installed to the secondary side of the transformer. A need exist for a switching power supply which uses only primary side information to regulate the output thereby reducing secondary side feedback devices. The invention disclosed herein will address the above problems.